For various household appliances it is not possible to switch off the whole appliance, due to convenience of use or technical reasons. For example, in video and audio equipment it is desirable that the device remains standby so that is capable of receiving and processing remote control signals. Or the device may have a permanently active control clock for activating other modules of the device at a predetermined time.
Due to regulations and the desire for green electronic designs, however, standby power consumption of appliances should be limited to a minimum.
Often a capacitive power supply is used to supply the part of the electronics inside the appliance that should remain permanently active. The appliance may for example be a coffee-maker or a washing-machine, having a micro-controller for controlling various functions of the appliance. A capacitive power supply has a lower cost-price than other power supplies, as for example a conventional transformer or a switch-mode power supply. Examples of a capacitive power supply are disclosed in US2006/0034109A1. In a capacitive power supply a capacitance functions as an impedance that causes a voltage drop from the main supply voltage to a voltage to be used by the application. As the current through the capacitance is 90 degrees out of phase with the voltage across the capacitance, the capacitive power supply itself consumes substantially no power. In practice it is difficult to design the capacitive power supply such that the phase shift is exactly 90 degrees, as account has to be taken of additional constraints. One thereof is that the inrush current of the device, occurring if the device is attached to the mains or switched on by a mains switch, is limited to value at which damage to the supply is prevented. In addition it must be taken into account that peak voltages may occur at the mains supply that may reach values to 2 to 3 kV. It must be prevented that this deviations ripple through too much as variations into the voltage supplied by the capacitive power supply, and therewith may cause undesired situations. In an appliance as referred to above this could occur for example if the variations in the voltage supplied by the capacitive power supply damage the microcontroller. Usually the capacitive power supply therefore includes an inrush current limiter, usually in the form of a series resistance that reduces the inrush current. As a negative side effect however this series resistance dissipates power.